The present invention relates to stock washers for removing impurities or other substances from a porous mat such as wood pulp. In particular, the present invention relates to hood enclosures for containing volatile chemical emissions from apparatus constructed and arranged for carrying out craft mill washing processes.
In a standard paper production line, wood chips are cooked with chemicals in aqueous solution, the precise composition of the cooking chemicals depending upon the desired resultant chemical mixture. The resulting chemical mixture, sometimes referred to as xe2x80x9cstockxe2x80x9d is composed of wood pulp and liquid containing residual chemical and dissolved woody materials, the liquid portion being commonly referred to in the art as xe2x80x9cred or black liquor.xe2x80x9d Separation of the pulp from the black liquor is normally carried out in a washing operation, after which the wash liquor is evaporated to recover the chemicals contained therein.
The most common type of washer system includes a rotary vacuum drum onto which the stock is spread. The drum is perforated, and a vacuum maintained inside causes the separation of the liquid from the pulp. The pulp assumes the form of a pulp mat which is still impregnated with chemicals and organics. A shower washer is usually positioned above the drum and typically extends axially along the drum and directs water at and through the pulp mat to remove these substances. A typical washing installation may include several washer drums in sequence with wash water being flowed against the flow of pulp movement so that the final washing stage uses clean water. Subsequent washing stages may be required if the washed pulp is to be bleached.
The liquor effluent from the washers comprises water, spent cooking solvents, and organic materials. The cooking solvents consist of sulfates (SO2) held in an aqueous solution, methanol/ODTT, and other toxic contaminants. In addition to the liquid contaminants, airborne contaminants are emitted from the washing process due to the high heat and steam introduced during the washing process.
As is customary in the industry, a hood is placed over the drum and washers to prevent steam from being lost and to protect workers from the washing chemicals. Access portals to the interior of the washing areas are incorporated in the hood structure to enable workers to clean the washing screens and provide general maintenance to the interior of the washer.
Current hood technology employed in the pulp and craft mill industries consists mainly of loose fitting metal covers with downward extending resilient flaps made of neoprene. While present hood technology may adequately protect workers from the hazardous liquid emissions of the washing operation, contemporary hoods do little or nothing to reduce exhaust gases which contain airborne chemical and toxic contaminants. Also, present hoods do not provide a means for establishing a negative hood pressure within the wash area to control out-gassing of contaminants and process evaporation.
As part of current environmental pollution controls, pulp mills incinerate toxic off-gases evacuated from their washing operations so that they can comply with EPA regulations. This incineration process necessitates that pulp mills incur additional manufacturing expense in the form of fuel consumption to incinerate the toxic gases. However, the EPA is moving toward more stringent regulations for paper mills; partly to encourage oxygen delignification (OD) implementation, which significantly reduces water usage and the emission of chlorinated pollutants to the atmosphere during the bleaching operation. Present gas incineration strategies will not provide sufficient pollution reductions to meet the new EPA standards. Furthermore, the published EPA goal for craft mills is a xe2x80x9cclosed millxe2x80x9d design with zero water discharge. In order for craft mills to meet the new EPA regulations soon to be promulgated, current hood technology must be improved to enclose pulp washers and contain the steam and vapor emissions. An additional benefit to the implementation of new hood technology will be to significantly reduce or eliminate the fuel costs for off-gassing incineration, as well as avoidance of EPA fines.
Therefore, there is a great need in the pulp mill industry for an improved hood enclosure that will permit maintenance access and the passage of various types of conduits into the interior of the washing unit while minimizing the emission of toxic substances into the environment. The hood must also be economical and resistant to the caustic chemicals used in washing systems.
It is the object of the present invention to provide a hood enclosure to cover and reduce the gaseous emissions from a pulp processing washing systems while maintaining a negative internal pressure within the wash system.
It is another object of the present invention to provide an economical and chemically resistant hood enclosure fabricated from Fiber Reinforced Plastic (FRP).
It a further object of the present invention to provide durable, renewable sealing methods for routine and overhaul maintenance access features as well as process and equipment penetrations from the washing system within the enclosure.
Briefly, the hood enclosure has a pair of end members, skirts along each side connect to the end members, and an upper spine connected at each end to the end members. A pair of clamshell style doors are mounted to the spine along longitudinal hinges and are opened vertically. A plurality of access doors are positioned within the skirts, and a plurality of spray or shower pipes penetrate the hood through an end panel to deliver wash liquid to the internal wash drums. Several sealing methods provide sealable access for the access doors, the clamshell doors, and the shower pipes to the interior area of the hood. Such sealing strategies include a peripheral neoprene gasket that abuts the clamshell doors, a raised comb on the hood skin having a U-seal and an elastomeric closure strap to bias access doors against the U-seal, and a saddle flange having a series of PET half rings correspondingly positioned with portals on an upper panel to receive a plurality of shower pipes that penetrate the hood. The pipes are further secured and sealed by a U-bolt which is seated in a groove in the PET half-rings and bolted to a continuous flange on the interior of the saddle flange. Neoprene gaskets backup the PET rings such that gaseous emissions are eliminated.